Fix null dereference in check_finalizer_nested if redirect malloc on …

…Linux

(a cherry-pick of commit 0408c6e from 'master')

Issue #582 (bdwgc).

As noted in GC_start_routine, an allocation may happen in
GC_get_stack_base, causing GC_notify_or_invoke_finalizers to be called
before the thread gets registered.

* pthread_support.c [!GC_NO_FINALIZATION && INCLUDE_LINUX_THREAD_DESCR
&& REDIRECT_MALLOC] (GC_check_finalizer_nested): If me variable is NULL
then return NULL; add comment.

pthread_support.c

@@ -734,8 +734,15 @@ GC_INNER GC_thread GC_lookup_thread(pthread_t id)
   GC_INNER unsigned char *GC_check_finalizer_nested(void)
   {
     GC_thread me = GC_lookup_thread(pthread_self());
-    unsigned nesting_level = me->finalizer_nested;
+    unsigned nesting_level;
 
+#   if defined(INCLUDE_LINUX_THREAD_DESCR) && defined(REDIRECT_MALLOC)
+      /* As noted in GC_start_routine, an allocation may happen in  */
+      /* GC_get_stack_base, causing GC_notify_or_invoke_finalizers  */
+      /* to be called before the thread gets registered.            */
+      if (EXPECT(NULL == me, FALSE)) return NULL;
+#   endif
+    nesting_level = me->finalizer_nested;
     if (nesting_level) {
       /* We are inside another GC_invoke_finalizers().          */
       /* Skip some implicitly-called GC_invoke_finalizers()     */